Add better information about time series and how to avoid generating too many

With k6 v0.41.0, some scripts that previously worked fine with millions of different time series will no longer be OK (https://github.com/grafana/k6/issues/2765).

We should add more warnings about that in the [Metrics](https://k6.io/docs/using-k6/metrics/) and [Tags and Groups](https://k6.io/docs/using-k6/tags-and-groups/) sections. Maybe polish a little bit the [URL grouping section](https://k6.io/docs/using-k6/http-requests/#url-grouping) and link it from more places, or move it to its own page? :thinking:

Start with the Metrics and Tags and Groups documentation pages, then review the URL grouping section in the HTTP requests documentation. Add warnings explaining how to avoid generating too many time series and improve the relevant cross-links; done means the guidance is present and the proposed pages are clearer to readers.

Written by the indexing model from the issue text.